Inrecentyears,therehasbeensignificantuseofironoxidenanoparticles(IONPs)with tailoredpropertiesfordevelopingnewinvivobiomedicalapplications.However,there arestillunansweredquestionsregardingtheincorporationofIONPsintothemetabolic cycle,includingtheeffectofmodifyingthesizeandfunctionalizationofthe nanoparticles,whichisessentialforspecificclinicaluses.Theexactprocessof agglomeration,degradation,andabsorption/clearancebythebodyisnotcompletely understood,particularlyhowtheseprocessesareaffectedbyphysicalandchemical propertiesofIONPs.Quantifyingthebiodistribution,transformation,andmetabolismof IONPsisanessentialstepinthedevelopmentoftheseapplicationsforclinicaluse.We proposetodevelopaquantitativemethodtodeterminethelongterminvivofateof IONPsbycombiningACsusceptibilitymeasurementswithmodelingofmagnetic relaxationandradionuclidelabeledSPECT/CTimaging.Theout-of-phaseAC susceptibilityprofileishighlydependentonthequantity,size,anddegreeof agglomerationoftheparticles,andACsusceptibilitymeasurementscandifferentiate betweennanoparticles,ironstorageproteins(ferritinandhemosiderin),and endogenousiron.Consequently,withinvivoexperimentsusingrodentmodels,this methodwillallowustoquantitativelydeterminethecompletebiodistribution, transformation,andparticokineticsofIONPsasafunctionoftheirsizeandsurface coating.Initialworkwillbeonparticlecorediametersoptimizedfortranslationalimaging inMRI(T1contrast)andtheemergingtechniqueofMagneticParticleImaging.Eachof thesewillbefunctionalizedwitheitherbiodegradable(polysaccharides)ornon- degradable(polyetheleneglycol)coatings,toevaluatetheeffectofsurfacecoatingson themetabolismandbiologicalfateofnanoparticles.SelectIONPswillalsobe radionuclidelabeledforSPECT/CTimagingforquantitativecomparisonandevaluation, whichwillprovideadditionalinformationaboutthedegradationoftheparticles.In summary,ourmethodwillprovideaprecise,quantitativedescriptionofthe particokineticsandultimatefateofIONPsinvivo,acrucialsteptowardsunderstanding theiroveralltoxicity,metabolism,andlong-termfate,withpotentialconsequenceinthe widerangeofclinicalapplicationswhereIONPsareadministered.